News Release Number: STScI-2005-32

Hubble, Sloan Quadruple Number of Known Optical Einstein Rings

The full news release story:

As Albert Einstein developed his theory of general relativity
nearly a century ago, he proposed that the gravitational
field from massive objects could dramatically warp space and
deflect light.

The optical illusion created by this effect is called
gravitational lensing. It is nature's equivalent of having a
giant magnifying lens in space that distorts and amplifies the
light of more distant objects. Einstein described gravitational
lensing in a paper published in 1936. But he thought the effect
was unobservable because the optical distortions produced by
foreground stars warping space would be too small to ever be
measurable by the largest telescopes of his time.

Now, almost a century later, astronomers have combined two
powerful astronomical assets, the Sloan Digital Sky Survey
(SDSS) and NASA's Hubble Space Telescope, to identify 19 new
"gravitationally lensed" galaxies, adding significantly to the
approximately 100 gravitational lenses previously known. Among
these 19, they have found eight new so-called "Einstein rings",
which are perhaps the most elegant manifestation of the lensing
phenomenon. Only three such rings had previously been seen in
visible light.

In gravitational lensing, light from distant galaxies can be
deflected on its way to Earth by the gravitational field of
any massive object that lies in the way. Because of this, we
see the galaxy distorted into an arc or multiple separate images.
When both galaxies are exactly lined up, the light forms a
bull's-eye pattern, called an Einstein ring, around the
foreground galaxy.

The newly discovered lenses come from an ongoing project called
the Sloan Lens ACS Survey (SLACS). A team of astronomers, led by
Adam Bolton of the Harvard-Smithsonian Center for Astrophysics
in Cambridge, Mass., and Leon Koopmans of the Kapteyn
Astronomical Institute in the Netherlands, selected the candidate
lenses from among several hundred thousand optical spectra of
elliptical galaxies in the Sloan Digital Sky Survey. They then
used the sharp eyes of Hubble's Advanced Camera for Surveys to
make the confirmation.

"The massive scale of the SDSS, together with the imaging quality
of the Hubble telescope, has opened up this unprecedented
opportunity for the discovery of new gravitational lenses,"
Bolton explained. "We've succeeded in identifying the one out of
every 1,000 galaxies that show these signs of gravitational lensing
of another galaxy."

The SLACS team scanned the spectra of approximately 200,000 galaxies
2 to 4 billion light-years away. The team was looking for clear
evidence of emission from galaxies twice as far from Earth and
directly behind the closer galaxies. They then used Hubble's
Advanced Camera for Surveys to snap images of 28 of these candidate
lensing galaxies. By studying the arcs and rings produced by 19 of
these candidates, the astronomers can precisely measure the mass of
the foreground galaxies.

Besides producing odd shapes, gravitational lensing gives astronomers
the most direct probe of the distribution of dark matter in
elliptical galaxies. Dark matter is an invisible and exotic form of
matter that has not yet been directly observed. Astronomers infer its
existence by measuring its gravitational influence. Dark matter is
pervasive within galaxies and makes up most of the total mass of the
universe. By searching for dark matter in galaxies, astronomers hope
to gain insight into galaxy formation, which must have started around
lumpy concentrations of dark matter in the early universe.

"Our results indicate that, on average, these ‘elliptical lensing
galaxies' have the same special mass-density structure as that observed
in spiral galaxies," Bolton continued. "This corresponds to an increase
in the proportion of dark matter relative to stars as one moves away
from the center of the lensing galaxy and into its fainter outskirts.
And since these lensing gelaxies are relatively bright, we can solidify
this result with further ground-based spectroscopic observations of the
stellar motions in the lenses."

"Being able to study these and other gravitational lenses as far back in
time as several billion years allows us to see directly whether the
distribution of dark [invisible] and visible mass changes with cosmic
time," Dr. Koopmans added. "With this information, we can test the
commonly held idea that galaxies form from collision and mergers of
smaller galaxies."

The Sloan Digital Sky Survey, from which the SLACS lens-candidate sample
was selected, was begun in 1998 with a custom-built ground-based telescope
to measure the colors and brightnesses of more than 100 million objects
over a quarter of the sky and map the distances to a million galaxies and
quasars. "This type of gravitational-lens survey was not an original goal
of the SDSS, but was made possible by the excellent quality of the SDSS
data," said Scott Burles of the Massachusetts Institute of Technology in
Cambridge, Mass., a SLACS team member and one of the creators of the SDSS.

"An additional bonus of the large size of the SDSS database is that we can
design our search criteria so as to find the lenses that are most suitable
for specific science goals," said SLACS team member Tommaso Treu of the
University of California, Santa Barbara. "Whereas until now we have
selected the largest galaxies as our targets, in the next stages of the
survey we are targeting smaller lens galaxies. There have been suggestions
that the structure of galaxies changes with galaxy size. By identifying
these rare objects 'on demand,' we will soon be able for the first time to
test whether this is true."

Added SLACS team member Leonidas Moustakas of the NASA Jet Propulsion
Laboratory and the California Institute of Technology in Pasadena, Calif.:
"These Einstein rings also give an unrivaled magnified view of the lensed
galaxies, allowing us to study the stars and the formation histories of
these distant galaxies."

The SLACS Survey is continuing, and so far the team has used Hubble to study
almost 50 of their candidate lensing galaxies. The eventual total is expected
to be more than 100, with many more new lenses among them. The initial
findings of the survey will appear in the February 2006 issue of the
Astrophysical Journal and in two other papers that have been submitted to
that journal.